One form of prior art multi-layer electroluminescent elements is shown in FIG. 4 in which a transparent electrode 2, a first insulative layer 3, an electroluminescent layer 4, a second insulative layer 5 and an opposed electrode 6 are accumulated in this order on a transparent glass substrate 1. The multi-layer electroluminescent element is configured to emit light when active ions in the electroluminescent layer 4 are energized by application of an alternating electric field of several decahertz to several kilo hertz approximately between the transparent electrode 2 and the opposed electrode 6. The multi-layer electroluminescent elements are used more and more for display of various devices.
In the prior art multi-layer electroluminescent elements, however, it is necessary to externally extend electrodes from upper and lower portions of the electroluminescent layer, and this electrode extension process is significantly complicated Along with an increased demand of electroluminescent elements for display of various devices, it is desired to improve their display resolving power. However, one of the electrode layers for extraction of electroluminescence (it is normally the electrode layer nearer to the transparent glass substrate) must be a transparent conductive layer which has the specific resistance of about 2.times.10.sup.-4 .OMEGA..cm as far as the present technical level permits. If the pattern width is decreased in the attempt to improve the display resolving power, its conductive resistance increases and fails to improve the display resolving power.
In this connection, the present inventor proposed a multi-layer electroluminescent element shown in FIGS. 5 and 6 (see Japanese patent applications 123880/1985, 132881/1985 and 123882/1985). The electroluminescent element includes a transparent conductive layer 12, an insulative layer 13, an electroluminescent layer 14, an insulative layer 15 and voltage applying electrodes 16 and 17 all accumulated in this order on a transparent glass substrate 11. The voltage applying electrodes 16 and 17 consist of at least one pair of electrodes which are not electrically connected. The voltage applying electrodes 16 and 17 overlap the transparent conductive layer 12. When an alternating voltage is applied between one pair of voltage applying electrodes 16 and 17, an electric field produced by the alternating voltage is applied between the voltage applying electrodes 16-17 and the transparent conductive layer 12 which forms an equivalent potential surface. As a result, the electroluminescent layer 14 between the voltage applying electrodes 16-17 and the transparent conductive layer 12 emits light. The light emitting portion of the electroluminescent layer 14 is shown by a hatching and designated by S in FIG. 6 where the voltage applying electrodes 16 and 17 overlap the transparent conductive layer 12.
The inventor's proposal, however, six layers excluding the transparent conductive layer 12 are interposed between the voltage applying electrodes 16 and 17. More specifically, three layers, i.e. the insulative layer 15, electroluminescent layer 14 and insulative layer 13, exist between the voltage applying electrode 16 and the transparent conductive layer 12, and three layers, i.e. the insulative layer 13, electroluminescent layer 14 and insulative layer 15 exist between the transparent conductive layer 12 and the voltage applying electrode 17. Therefore, a significantly large voltage is required between the voltage applying electrodes 16 and 17.
In this connection, the present inventor further proposed a multi-layer electroluminescent element shown in FIGS. 7 and 8 (see Japanese patent application 7014/1986). The multi-layer electroluminescent element includes a transparent conductive layer 12 provided on a transparent substrate 11, an electroluminescent layer 14 provided on a part of the transparent conductive layer 12, a first voltage applying electrode 16 provided above the electroluminescent layer 14 via an insulative layer 15, and a second voltage applying electrode 17 provided above a part of the transparent conductive layer 12 via the insulative layer 15 and not overlapping the electroluminescent layer 14. When an alternating voltage is applied between the first and second voltage applying electrodes 16 and 17, an electric field produced by the alternating voltage is applied between the first and second voltage applying electrodes 16-17 and the transparent conductive layer 12 which forms an equivalent potential surface. As a result, the hatched portion S of the electroluminescent layer 14 at which the first voltage applying electrode 16 overlaps the transparent conductive layer 12 emits light.
The multi-layer electroluminescent element requires a decreased driving voltage because the number of layers is decreased between the first and second voltage applying electrodes 16 and 17. However, the decrease of the driving voltage is still insufficient. Further, since the transparent conductive layer 12 is spaced from the second voltage applying electrode 17 by only one layer, i.e. the insulative layer 15, it causes a leak between them which often damages the element.